1. Field of the Invention
This invention relates generally to the field of lifts and more particularly to self-stabilizing hoists and suspension systems such as those for suspending and lowering and lifting watercraft into and from the water.
2. Description of Related Art
Mechanisms for lifting, lowering and suspending large loads have been in use for centuries. In particular, pulley and cable systems are especially useful because they are light, strong, adaptable to many loads and easily powered by a motor or a manually driven winch. In lifting large loads, it is often inconvenient or impossible to provide for cables or structural members that pass overhead across the load. This is particularly true of large loads, such as vessels or machinery, or in mobile applications in which it is difficult or expensive to transport a large overhead structure to a remote location.
In addition, it is desirable in most applications to lift and lower the load without allowing it to tilt, which can affect liquids in the load, and which can move the center of gravity of the load to one side or the other, rendering the system unstable.
In view of the foregoing, it is self-evident that a simple mechanism that lifts and lowers large loads without side-to-side rotation, and without the use of cables or structural members that pass over the load, has significant benefits. Several patents have attempted to address this need.
U.S. Pat. No. 7,070,171 is directed to a suspension and hoisting system including two cable circuits, each including a cable fixed at one end and attached to as lifting apparatus, such as a winch, at the other, and being reeved around deflection pulleys mounted on two parallel shafts located at either end of the load support, and with the lifting apparatus of each cable being mounted above the load support on the side of the load that is opposite its fixed end. The cable of at least one of the cable circuits is reeved successively around at least two coaxial deflection pulleys mounted on each end shaft in such a way that on at least one of the shafts at least one pulley of one cable circuit rotates in the same direction as at least one pulley of the other cable circuit under identical lifting and lowering action of the two lifting apparatus, while these two pulleys are constrained to rotate in opposite directions by any downwardly oriented force independent of the action of the lifting apparatus. Accordingly, these two pulleys are interlocked to reduce or eliminate any tendency of the load support to incline under the effect of this downward force.
This '171 patent suffers from reliance on friction between cable and pulley to prevent the downward force from lowering one end of the load. In many applications, maintaining a friction-free contact between pulley and cable is difficult because of the presence water, grease, and dirt on the cables. Moreover, the two cable circuits are of unequal length, which results in unequal elongation under temperature variations, which will tend to make one or the other cable prone to slipping.
U.S. Pat. No. 5,427,471 is directed to a boat lift, including a vertical main post mounted on dock with the lower end mounted in the water but spaced above the floor of the water. A frame includes an outer guidepost having a base mounted on the water floor with an upper end above the water level; the frame connected by a single member to the main post. A platform unit for supporting a small boat is slidably connected at both ends to the main post and the guide post, and a cable and winch unit is interconnected among the foregoing elements for effecting a level lifting and lowering of the platform unit into and out of the water. This patent requires one end of the cable to be secured at the bottom of the main post, at which point it is continually immersed in the water. This requires frequent replacement of the cable, or the use of a non-corrosive cable material, which adds considerably to the cost of installation and maintenance.
U.S. Pat. No. 6,640,736 is directed to a synchronously driven, multiple cable boat lift used in combination with proximal and distal support structures located on respective sides of a boat to selectively lift and lower the boat out of and into a body of water. The boatlift includes a motor mountable on the proximal support structure and being selectively driven in opposing first and second directions. A boat accommodating platform is located between the proximal and distal supporting structures. A plurality of lift cables operably interconnect the motor and the platform for synchronously raising and lowering the platform. This patent does not provide a mechanism to prevent the boat from rotating as it is raised or lowered, and is limited to small craft because of the configuration of the lifting beams.
U.S. Pat. No. 5,090,841 is directed to a boat lift in which a hydraulic pump and cylinder with a piston are mounted on a manual boat lift such that the piston rod carries a pulley which engages the lifting cable so that, when the piston moves in the cylinder, the boat will move up or down. This patent provides a motion stabilized against rotation for one side of the boat. It does not provide for stabilization against accidental roll of the boat. Further, it requires that part of the cable be immersed in the water while the boat is in the lifted position, which has the same disadvantages as U.S. Pat. No. 5,427,471. Moreover the invention calls for a piston and cylinder in addition to lines and pulleys.
U.S. Pat. No. 4,401,335 is directed to a boat hoist including a lift suspension system which results in even application of forces to points on the movable platform remote from the point of attachment of the lifting device, thus eliminating twisting of the loaded platform. This patent achieves equal loading of lift points by means of a cable arrangement similar to that of U.S. Pat. No. 5,427,471, which requires that one end of each of two cables be fixed below the surface of the water.
It is seen that all the above load lifting patents provide raising and lowering of a large load without cables or structural members passing over the load, but either allow de-stabilizing rotation of the load, or have one or more substantive disadvantages such as requiring mechanisms substantially more complex than a pulley and cable system, or restricting the location and type of pulleys and cable fastenings, or depending upon frictional resistance to rotation of the load.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those skilled in the art upon a reading of the specification and a study of the drawings.